Compressible fluid velocity measuring instrument

Abstract

A velocity measuring instrument of the type having a rotatable, counterweighted, spring-controlled vane and pointer assembly in which the outlet aperture for the fluid is contoured exponentially or logarithmically to present an exit flow area that varies as a function of vane position for linearizing the movement of the vane and pointer assembly in response to the pressure of the fluid on the vane. In one embodiment a true logarithmically varying area port is provided. In other embodiments, two or more openings of varying area or geometry are configured to approximate a logarithmic outlet aperture. A baffle may be provided on the exterior of the case to prevent fluid from entering the outlet aperture, and the interior of the case may be contoured adjacent to the outlet aperture and along the path of the vane to provide a variable leakage path clearance between the vane and the case. A grease-coated surface that contacts the vane and pointer assembly provides viscous damping to prevent pointer oscillation.

What is claimed is: 
     
       1. A velocity measuring instrument comprising: a case for immersion in a flowing fluid and having an entrance aperture for the entrance of said fluid, a variable flow area outlet aperture for the exit of said fluid, and a window;   a lightweight, counterweighted vane assembly pivotally mounted for rotation within said case and having a vane disposed adjacent said entrance aperture and responsive to said fluid entering therethrough, and having a pointer adjacent said window; and   a spring coupled to said vane assembly for controlling the movement thereof;   said variable flow area outlet aperture being contoured to vary the flow area of said outlet aperture substantially exponentially as a function of the position of said vane assembly for linearizing the movement of said vane assembly in response to the pressure of said fluid on said vane;   the inside of said case being contoured on both sides of said outlet aperture along the arc described by said vane as said vane assembly rotates to provide a variable clearance between said vane and the inside of said case to permit a larger area aperture at the high flow rate end.   
     
     
       2. A velocity measuring instrument comprising: a case for immersion in a flowing fluid and having an entrance aperture for the entrance of said fluid, a variable flow area outlet aperture for the exit of said fluid, and a window;   a lightweight, counterweighted vane assembly pivotally mounted for rotation within said case and having a vane disposed adjacent said entrance aperture and responsive to said fluid entering therethrough, and having a pointer adjacent said window; and   a spring coupled to said vane assembly for controlling the movement thereof;   said variable flow area outlet aperture being contoured to vary the flow area of said outlet aperture substantially exponentially as a function of the position of said vane assembly for linearizing the movement of said vane assembly in response to the pressure of said fluid on said vane;   the inside of said case being contoured on one side of said variable flow area outlet aperture along the arc described by said vane as said vane assembly rotates to provide a variable flow area between said vane and the inside of said case that is a function of vane position to permit a larger area outlet aperture at the high flow rate end.   
     
     
       3. A velocity measuring instrument comprising: a case for immersion in a flowing fluid and having an entrance aperture for the entrance of said fluid, a variable flow area outlet aperture for the exit of said fluid, and a window; a lightweight, counterweighted vane assembly pivotally mounted for rotation within said case and having a vane disposed adjacent said entrance aperture and responsive to said fluid entering therethrough, and having a pointer adjacent said window; and   a spring coupled to said vane assembly for controlling the movement thereof;   said variable flow area outlet aperture being contoured to vary the flow area of said outlet aperture substantially exponentially as a function of the position of said vane assembly for linearizing the movement of said vane assembly in response to the pressure of said fluid on said vane;   said vane assembly being provided with a non-linear viscous damper which provides high damping at high flow rates, and low damping at low flow rates, said non-linear damper comprising a damping plate coated with grease and mounted in close proximity to said vane assembly and disposed so as to engage the surface of said vane assembly with an area that increases with increasing deflection of said vane assembly.   
     
     
       4. A velocity measuring instrument comprising: a case for immersion in a flowing fluid and having an entrance aperture for the entrance of said fluid, a variable flow area outlet aperture for the exit of said fluid, and a window;   a lightweight, counterweighted vane assembly pivotally mounted for rotation within said case and having a vane disposed adjacent said entrance aperture and responsive to said fluid entering therethrough, and having a pointer adjacent said window; and   a spring coupled to said vane assembly for controlling the movement thereof;   said variable flow area outlet aperture being contoured to vary the flow area of said outlet aperture substantially exponentially as a function of the position of said vane assembly for linearizing the movement of said vane assembly in response to the pressure of said fluid on said vane;   said entrance aperture being located to be upstream of said outlet aperture when said case is immersed in a flowing fluid;   a baffle being provided on the exterior of said case adjacent to and partially surrounding said outlet aperture, said baffle being closed toward the upstream side and toward the entrance aperture, and being open toward the downstream side, said baffle extending over said outlet aperture but spaced away therefrom to prevent the entry of fluid through said outlet aperture when said case is completely immersed in a flowing fluid with varying angles of attack to the entrance aperture.